The present invention relates to a method for monitoring the enabling of a system which is assigned to an internal combustion engine, in which the readiness for operation of the system is dependent on an operating temperature.
In order to reduce nitrogen oxides in the exhaust gas of an internal combustion engine, use is already being made of so-called SCR catalytic converters (Selective Catalytic Reduction) in the exhaust section of the internal combustion engine to reduce the nitrogen oxides contained in the exhaust gas to nitrogen in the presence of a reducing agent. Ammonia is required for the reaction to take place, and this is added to the exhaust gas. In general, an aqueous urea solution is used for this purpose, being injected into the exhaust line upstream of the SCR catalytic converter with the aid of a metering device. Ammonia is formed from this solution and acts as a reducing agent.
For optimum aftertreatment of the exhaust gas, the reducing agent is injected into the exhaust line as required. The metered quantity is calculated in accordance with NOx emissions upstream of the SCR catalytic converter. This calculation includes the reducing agent pressure, which is adjusted to a predeterminable desired pressure.
Since the metering system for the SCR catalytic converter is a component of significance for the exhaust gas, it is necessary to monitor the ability to function of the metering system. The current practice is to monitor the time up to the closing of the control loop (Time To Closed Loop—TTCL) of the SCR metering system or to the enabling of this system on the basis of an evaluation of the temperature of the SCR catalytic converter and the readiness for metering of the SCR system. During this process, a check is made to determine whether the temperature of the SCR catalytic converter is above an applicable threshold and whether the SCR system is ready for metering. If both conditions are satisfied and metering is not active after the expiry of an applicable time, a fault is set. The temperature of the SCR catalytic converter is calculated on the basis of a signal from a temperature sensor arranged upstream of the SCR catalytic converter. The signal from this temperature sensor is generally denoted by T6. In order to ensure that the information provided by the TTCL monitoring is reliable, it is necessary to monitor the functioning of the temperature sensor or the signal T6 from said sensor. Normally, the plausibility of T6 is checked by comparison with a model temperature, which is, in turn, calculated from the signal from another temperature sensor fitted in the exhaust system upstream of the temperature sensor mentioned. The signal from this further temperature sensor in the exhaust system is generally denoted by T5.
Since temperature sensors are generally subject to tolerances, it is not possible to rule out a certain inaccuracy in the plausibility checking and monitoring. It is therefore the underlying object of the invention to provide improved monitoring of the enabling of a system assigned to the internal combustion engine, in particular for the operation of an SCR metering system.